The present invention relates to valves in general, and more particularly to improvements in fluid flow regulating valves (especially volumetric fluid flow regulating valves). The invention also relates to improvements in methods of regulating the flow of fluids, for example, to the fluid-consuming or fluid-operated components of power trains in motor vehicles.
A continuously variable transmission (CVT) of the type often utilized in the power trains of motor vehicles normally comprises several fluid-operated components, e.g., hydraulic cylinder and piston units for the adjustable flanges of pulleys, for torque sensors, for lubricating devices and/or others. Reference may be had, for example, to commonly owned U.S. Pat. No. 5,711,730 granted Jan. 27, 1998 to Oswald Friedmann et al. for "TORQUE MONITORING APPARATUS".
Volumetric fluid flow regulating valves (hereinafter called flow regulating or flow control valves for short) are utilized when the flow of a working fluid (e.g., oil) to a hydraulic cylinder or another consumer of hydraulic fluid must be influenced to shift, or to permit shifting of, the position of a piston or another mobile valving element. For example, it is often desirable or necessary to divert a given percentage of oil from a stream which is being made available by the outlet of an oil pump, and to convey the diverted percentage to a consumer in a transmission, whereas the remainder of the oil stream is compelled to circulate, e.g., along an endless path defined, for example, by a pump circuit.
As a rule, a flow regulating valve has a well-defined so-called no-regulation point, namely a condition when the pressure of hydraulic fluid being supplied to a consumer remains at least substantially constant regardless of whether the quantity (volume) of pressurized fluid being supplied by the outlet of the pump continues to increase.
A standard flow regulating valve normally comprises a housing or body for a normally piston-shaped valving element which is reciprocably confined in an elongated passage (e.g., a bore or hole) of the housing. A spring reacts against the housing at one end of the passage to bear upon the adjacent end portion of the piston and to thus urge the entire piston toward the other end of the passage. A flow restrictor is employed to establish a difference or to change the difference between the fluid pressures at the two ends of the valving element (hereinafter piston for short), e.g., when it becomes necessary to change the axial position of the piston relative to the housing. The latter has an outlet which receives fluid with assistance from or against the resistance of one or more valve springs when the quantity of pressurized fluid being supplied to the inlet by the pump exceeds the requirements of one or more consumers, i.e., the piston then assumes an axial position in which the consumer or consumers continues or continue to receive an optimum quantity of pressurized fluid whereas the surplus is caused to circulate along an endless path which can be defined in part by a source of fluid (e.g., a sump or another suitable reservoir) for the pump.
The aforementioned no-regulation point constitutes an important parameter because the volumetric flow of hydraulic fluid to the consumer(s) must be considered in designing the entire system (such as the power train in a motor vehicle). The volumetric flow constitutes an input parameter. Any changes (shifting) of the input parameter can exert a negative influence upon the operation of the system, primarily because a shifting of the input parameter prevents the establishment of an optimum or desirable volumetric flow.
Furthermore, due to the characteristics of the mode of operation of the consumer or consumers which receives or receive hydraulic fluid from the volumetric fluid flow regulating valve, the pressure of fluid reaching the consumer(s) can exceed a threshold value in response to shifting of the aforementioned no-regulation point. This causes the flow restrictor to initiate the establishment of a certain pressure drop. This, in turn, results in the establishment of an excessive pressure differential between the chamber for the aforementioned spring or springs at one axial end, and a second chamber at the aforementioned outlet (to the fluid circulating arrangement) of the housing in spite of the pressure drop which is caused by the flow restrictor, when the aforementioned pressure differential exceeds a certain value. This results in the establishment of a flow of leak fluid from the chamber for the spring(s) at one axial end of the piston and the outlet lading to fluid circulating arrangement.
The fluid which leaks from the spring-containing chamber is part of the fluid which is intended to reach the consumer(s). Such leakage results in a shifting of the operating point, and the end result is a change in the operation of the consumer or consumers.
If the quantity of fluid being supplied by the pump to the regulating valve frequently and greatly exceeds the quantity of fluid that is required for proper operation of the consumer(s), the regulating valve must carry out appropriate adjustments between the quantity of the fluid flowing to the consumer(s) and the quantity of fluid which is being recirculated along a predetermined path at frequent intervals. If the regulating valve is utilized in the power train of a motor vehicle, such frequent adjustments take place when the pump which supplies pressurized fluid to the valve is driven by the prime mover (e.g., a combustion engine) of the motor vehicle, the valve must be adjusted (more specifically, the quantity of fluid which is fed into the endless path must be changed) in response to each increase (or each pronounced increase) of the engine RPM. Thus, the piston is compelled to perform frequently recurring oscillatory or reciprocatory movements in the passage defined by the housing of the regulating valve. Since the piston and the valve body are normally made of a metallic material, frequently recurring movements of the piston entail the development of pronounced wear, especially at the locations where the piston is to sealingly engage the surface bounding te passage in the valve housing. The development of wear upon the piston and/or upon the valve housing aggravates the situation because it leads to a more pronounced leakage of fluid from the spring-confining chamber to the outlet of the pump housing.
Attempts to reduce the likelihood of wear upon the piston and/or upon the housing of the regulating valve, or to reduce the extent of such wear (e.g., by appropriate shaping (bevelling or rounding) of the sealing portions of the piston and/or the valve housing) have met with limited (if any) success. One of the reasons is that, as a rule, the conveyed and recirculated fluid (such as oil) normally contains solid particles that are likely to be intercepted in the narrowest portion or portions of the unavoidable clearance(s) between the periphery of the piston and the adjacent surface(s) bounding the passage of the valve housing and receiving the reciprocable piston. The intercepted solid particles are an important (even primary) cause of extensive wear, particularly upon the specially configurated (such as the aforementioned rounded or bevelled) surfaces at the locations where the piston is to sealingly engage the valve housing. Excessive bevelling, rounding and/or analogous treatment of those portions of the piston and/or the valve housing where such portions are to sealingly engage or approach each other can result in a pronounced tendency of the piston to change its orientation (particularly to perform a tilting movement) relative to the valve housing. Of course, such changes of orientation of the piston in the bore or passage of the valve housing even further increase the likelihood of excessive wear, i.e., of increased flow of leak fluid from the spring-containing chamber to the outlet of the valve housing.
The just discussed tendency of the piston to change its orientation relative to the valve housing can be counteracted (to a certain extent) by increasing the axial length of the piston. However, the axial length of the piston cannot be increased at will (namely to the extent which is necessary to prevent any (or any appreciable) changes of orientation of the piston in the passage of the valve housing), especially in the power trains of motor vehicles wherein the space allotted for the transmission (such as a CVT) and its hydraulically operated constituents or auxiliary equipment) is at a premium.